Dexter's Inventions

Dexter, the wannabe inventor, has come up with what seems to be a great invention.
It's a special lock that is fitted on your bathroom door. Every time you enter the bathroom and shut the door behind you, it automatically locks. When you want to leave, you just pull on the handle and the door will open!
Although this seems like a good invention, there is a design flaw that would stop people from buying this product.

What is it?

Answer:

There are two reasons why this product wont sell:

1: If no one is in the bathroom and the door closes, no one can get in.

2: If you're in the bathroom and for some reason you can't get the door, you're stuck because no one could help you.